1. Field of the Invention
The present invention relates generally to press vibration monitoring and more particularly, to a method of generating a press load/speed vibration severity capacity indicator for the determination of press/die long-term operating reliability during production operation and to an apparatus utilizing the information generated by the above method in monitoring press vibration severity.
2. Description of the Related Art
The traditional method for calculating the tonnage of a press die is mainly by a means of static load calculation. A given die has a certain material shear length and a stock material with a certain thickness. From this, the tonnage of the die or the force necessary to shear or form the part may be calculated. Traditional press sizing has been based on "static 11 die shear loading as calculated using the equation; [Shear Length (in.) [Thickness (in.)] [S. (lb/in2)]=Shear Load (lb).
This load (plus forming and blanking static loads) has traditionally been considered the only significant load and thus the peak dynamic load of the press. Generally, on shorter machines at speeds below 300 strokes per minute, dynamic effects are not a major influence on die application severity. As press speeds are increased however, there are several other dynamic influences which become present, thereby creating additional press loadings in addition to increases produced by the actual shear loading above the traditional static calculated value. In many cases, these dynamic loads surpass the shear load as the peak dynamic load. In addition to greater effective shear loads, additional impact forces are created as press speed increases, which further contributes to the vibration of the press structure.
It has been found through experimentation, that as the press speed increases there are impact magnifications to static loads as well as several additional loads that occur that are not present at slower press speeds. There are actually several different sources of additional die load parameters that many press operators, production managers, or owners do not necessarily know exist. At higher speeds, even though not exceeding the capacity of the press, the press requires more force to make the part, which in turn creates a different set of more severe vibration conditions.
At higher press speeds, in the press structure, the loads are applied much more quickly, are released more quickly, and in general are producing a much stronger shock wave which is dispersed and dissipated through the press structure. By increasing the speed of the press, the slide velocity at any given point above the bottom dead center position is increased, thereby increasing the impact forces of the punches on the stock material. These impact force increases are related to the square of the velocity. Therefore, press speed is one of several factors increasing vibration in the press. By running the press at higher speeds, more severe vibration is transmitted through the press.
A second factor contributing to press vibration is the stroke length, which increases the impact forces and loading on the press. A third factor is the contact distance of the die punches and stripper plate above bottom dead center. The higher these components contact above bottom dead center, the greater the impact velocity and, therefore, the more severe the vibration level.
Another factor relative to press vibration increase is the stored energy released during the manufacture of the part. Deflections occur on the press structure during loading of the die. As the stock material fractures through, called snap through, the release of the stored deflection energy sends a vibration shock wave through the press structure. The released stored energy also has the ability to accelerate the slide downward, which can cause the die punches to penetrate the stock material more deeply. As the applied load increases, so does the stress and deflection levels within the press structure, therefore causing increased energy release and increased vibration.
Yet another factor which affects the press structure and vibration is the use of flattening stations or stop blocks. If these devices are utilized in the die, then additional loads and impact forces are present. As press speed increases, the press shutheight will naturally close in, which, if stop blocks are utilized, will cause a larger load to be applied. The press shutheight naturally closes in as press speed increases due to the inertia forces developed.
Still another factor is the thermal shutheight effect. Again, as speed is increased, there is a viscous shear of the oil within the press crankshaft and other bearing clearances. The heat generated from the shear of the oil is conducted through the press structure and drive connections, causing the shutheight to dimensionally close in more deeply.
Thus, the above described dynamic effects that occur during press operation increase the loading and overall vibration levels induced in the press structure, all of which increase with an increase in press speed.
Vibration stress magnifications, created by dynamic load increase, can cause many problems to press structures. Cracks can develop over time in the castings anywhere within the press structure or its parts if long term dynamic load increases are unknown or go ignored. Broken structural and component parts such as tie rods, crankshafts, crowns, slides and dynamic balancers have been reported, and in all instances the vibration severity has been able to be correlated by field service failure data to develop specific threshold vibration severity levels measured on the press structure during production. At certain definable vibration severity levels, stress magnification levels will be present thus creating increased maintenance severity problems for the press.
The relative life of a press is thus determinable from the accumulative effects of the vibration severity levels experienced over this period of time. A press may withstand high vibration levels without major structural damage if the duration period is relatively short. Also, a press will certainly withstand low vibration levels without structural damage no matter what the duration period.
Accumulative structural damage will occur, however, when a press is run in a magnified stressed condition as a result of medium to high vibration severity levels over a longer duration period whether run continuously or intermittently. The damage will not necessarily be evident in the early stages but will begin to appear over time.
Vibration monitoring systems of the prior art require that a no load response level be determined with periodic no load checking of the relative level at several specific component locations, to try to evaluate the progress of component deterioration.
What is needed in the art is a portable apparatus which measures the actual application vibration severity levels while in actual production, which allows the press operator, tooling engineer, production manager, or owner to know the long term reliability effects of running the press at any combination of sensed speed and load, by monitoring the actual vibration severity level of the die application via measurement of press RMS velocity by means of an accelerometer, and comparing the corresponding operating vibration severity level to a vibration severity zone chart.